virtio-blk: Fix indentation for vring_get_buf call

Adjust the indentation of the vring_get_buf function call to
maintain consistency with surrounding code.

Fix warning:

In file included from out/ccode32flat.o.tmp.c:83:
./src/hw/virtio-blk.c:56:9: warning: misleading indentation; statement is not part of the previous 'while' [-Wmisleading-indentation]
   56 |         vring_get_buf(vq, NULL);

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>